In general, a television display of an NTSC broadcasting system forms an image frame with two field signals, and one frame is generated from 525 line signals which are contained in an image signal. The image signal also comprises a vertical synchronous signal V-SYNC for synchronizing the two field signals with each other and contains a horizontal synchronous signal H-SYNC for synchronizing the 525 line signals with each other.
In an image signal recording/reproducing apparatus, the synchronous signals V-SYNC and H-SYNC are detected and utilized for various signal processing operations. For example, if a synchronous signal is not detected from an image signal that is input when a television is switched from a broadcasting channel to another broadcasting channel, the apparatus determines that no image signal exists. In such instance, a noise picture, which is normally displayed when a synchronous signal is not detected, is replaced by a blue background picture or another reference picture. Thus, tuning to a channel which is not carrying an image signal is more aesthetically pleasing to a viewer.
In a conventional method for detecting an image signal, the input of an image signal is detected by counting the number of synchronous signals generated during one field or one frame of the image signal or by determining the cycle of the synchronous signal. However, various noises exist in an image signal recording/reproducing apparatus. Thus, when the frequency (or pulse width) of the noises are similar to the frequency (or pulse width) of the synchronous signals, the noises can be misjudged as synchronous signals. (See FIG. 7A). As a result, the method incorrectly detects the presence of an image signal. Similarly, if noise significantly interferes with a valid synchronous signal, the method may incorrectly detect the absence of an image signal.